Such high voltage test instruments are used in particular for what are known as VLF (Very Low Frequency) testing of cables, wherein a cable to be tested for damage is subjected to an a.c. voltage—which is selected to correspond to the type and length of the cable to be tested—in a range from a few kV to several tens of kV or even 100 kV at a low frequency of preferably 0.1 Hz. In this way it is possible to test cables having lengths of several kilometers for damage to their insulation or the like. However, high-voltage test instruments of the class in question are also suitable for testing other electrical components or subassemblies, such as capacitors, resistors, transformers, motors, generators, switches or relays. Likewise, however, a high-voltage test instrument of the class in question can also be designed for a d.c. voltage test or what is known as a 50-Hz test.
The test voltage necessary for test operation is produced by the electronic unit of a test instrument of the class in question, preferably amplifying or transforming an input voltage present at a terminal for an external voltage supply by means of an amplifier operating in what is known as class-AB mode. The term electronic unit is to be understood broadly in the present context, since a test voltage suitable for the intended application can be produced even by means of simple circuit arrangements (for example, comprising at least one switch and one resistor). Even a portable voltage generator can be used as the external voltage supply, especially for measurements in the field. However, battery-powered operation of a high-voltage test instrument of the class in question is also conceivable, and so any terminal that may be present for an external voltage supply can be used for recharging the battery.
High-voltage test instruments of the class in question also have a control area, which preferably has not only some control elements for selecting the test voltage or the time variation of the test voltage in the manner of a test program but also a display and/or other indicating elements, with which the functions of the instrument and the result of the test can be monitored.
Compared with other portable technical instruments, however, the electronic unit of high-voltage test instruments of the class in question generates an extreme amount of heat, since, depending on the particular design of the electronic unit, large heat losses on the order of magnitude of 20 W up to several kW occur and have to be dissipated as waste heat during high-voltage amplification or switching. The cooling system must therefore meet particularly exacting requirements, and this proves to be particularly difficult, especially against the background that increasingly more compact and lightweight instruments are desired for reasons of the greatest possible portability.
The present invention therefore relates to high-voltage test instruments in which the cooling system is provided with an air-cooled assembly having an air inlet and an air outlet, wherein the air-cooled assembly either already represents the primary cooling system of the high-voltage test instrument or is a secondary system, such as for a liquid-cooled assembly and especially an oil-cooled assembly. In the present case the cooling system is integrated into the same housing of the high-voltage test instrument in which the electronic unit of the test instrument is also permanently accommodated, while in the prior art there are usually provided openings as air inlet and/or air outlet in one or more side walls of the housing.
In high-voltage test instruments cooled primarily by air, it is to be taken into consideration that particularly exacting requirements must be imposed in this case also on the cooling members or on the connection thereof to the electronic unit. In this regard, reference is made to PCT/AT 2006/000510, where there is described a suitable arrangement of cooling members for a high-voltage test instrument of the class in question.
In the case of portable use, the high-voltage test instruments described in the foregoing are carried to the respective cable or component to be tested and then started up on the spot. Under these conditions, special care is to be taken that the electronic unit of a high-voltage test instrument of the class in question is adequately protected against ingress of dirt or dust and/or against ingress of water, especially also during transportation.
Heretofore a separate transport housing, for example in the form of a carrying case or a carrying bag, from which the high-voltage test instrument must be removed before being started up, has been provided for this purpose. In view of the weight thereof, often several tens of kg, this is indeed cumbersome for the instruments of the class in question, but is nevertheless necessary in order to have free access to the terminals of the high-voltage test instrument and also to keep the air inlet and air outlet unobstructed, since otherwise the instrument would rapidly overheat.
For various technical instruments, however, there are already known, for example from U.S. Pat. No. 5,631,570, 5,803,603 and 4,998,067, instrument housings that permanently accommodate the electronic unit and offer some protection against the potential ingress of water or dust. None of these, however, are sufficiently suitable for transportation and/or operation of the respective instrument under adverse environmental conditions, such as rain.